Improving eyesight is vitally important. Precise measurement of the eye""s physical characteristics, including features of the eye, in order to prescribe vision correction is also vitally important.
With the advent of new technologies capable of creating more complex optical surfaces, a resurgence of interest has arisen in the tools required to measure the eye""s optical characteristics to a higher degree of complexity than was possible before.
This invention is an improvement on a system described by U.S. Pat. No. 5,963,300 to Horwitz. In the Horwitz system, a light beam is projected into the eye. The light beam is of a diameter equal to or larger than that of the eye""s pupil. The eye focuses the light beam onto the retina, the beam then reflects back out of the eye, through the optical components of the eye. A relay lens system collects the light reflected from the eye, projecting the collected light through a reticle, or a plurality of reticles. A spatial filter (an iris), is positioned within the relay lens system to block unwanted reflected light. The light that passes through the reticle(s), is projected onto a translucent screen to create a image on the screen. A charged coupled device CCD camera is focused onto the screen to xe2x80x9cseexe2x80x9d the patterns created by the reticle(s). A computer is used to convert the CCD camera images to digital data. The computer then analyzes the data to determine the refractive condition of the eye. The computer also analyzes the position of the reflected glint from the vertex of the cornea as well as the position of the pupil, compares the two positions, and determines where the eye is gazing.
There are several methods available to measure the reflected wavefront of an eye. The method of this invention is known as xe2x80x9cTalbot/Moire Interferometry.xe2x80x9d Of the other methods available, the most common method is known as a xe2x80x9clenslet array systemxe2x80x9d,or a xe2x80x9cHartmann Shackxe2x80x9d sensor. Such a wavefront sensor is described by Liang et al. in xe2x80x9cObjective Measurement of Wave Aberrations of the Human Eye with the Use of a Hartmann-Shack Wave-Front Sensor,xe2x80x9d Journal of the Optical Society of America, Vol. 1, No. 7, July 1994, p.p 1949-1957.
One of the earlier Hartmann Shack systems is described by U.S. Pat. No. 5,949,521 to Williams. A light beam is projected into the eye. Williams first passes the light through optical components and then reflects it from a deformable mirror before projecting it into the eye. A relay lens system collects the light reflected from the eye, projecting the collected light onto a deformable mirror, which in turn reflects it to a lenslet array.
A lenslet array is a disc with many, many tiny lenses, much like an insect eye, but flat instead of spherical. The lenslet array creates numerous spots of light focused into aerial images. If the light being collected by a tiny lens approaches the lens xe2x80x9cstraight onxe2x80x9d,then the spot that the tiny lens forms will be along the optical axis of the tiny lens. However, if the light is approaching the tiny lens not xe2x80x9cstraight onxe2x80x9d, but skewed off to one side of the optical axis, then the resulting spot will be formed to one side of the optical axis of the tiny lens. When the reflected light emerging from an eye being analyzed is not perfectly aligned along the optical axis, then the eye has a defect in it. The resulting shift in the position of the spot formed by the tiny lens indicates the type of, and the degree of the defect in the eye. The positions of each of the tiny lenses are related to the optical performance of the corresponding position of the eye. In other words, a tiny lens at the very top of the array that is collecting light emerging from the eye will produce a spot, and subsequent information about the light that emerged from the top of the eye. Conversely, a spot on the bottom of the array corresponds to the bottom of the eye, and so forth. (If the image is inverted or mirrored, as it sometimes is depending upon the optical design, then the relationship must be adjusted accordingly. For example, if the image is inverted, then the xe2x80x9ctopxe2x80x9d of the eye will be represented by a spot on the xe2x80x9cbottomxe2x80x9d of the array.) A CCD camera is focused onto the aerial plane where the spots come into focus, xe2x80x9cseeingxe2x80x9d the spots of light. A computer is used to convert the CCD camera images to digital data. The computer then analyzes the data to determine the refractive condition of the eye, by comparing the shift of each spot from where the spot would have been had the eye been defect free. The computer changes the shape of the deformable mirror to alter the resulting spot pattern produced by the lenslet array, attempting to alter it in such a manner to bring the spots closer to the position where the spots would have been for a properly focusing eye.
An improvement on the Hartmann Shack system is described by U.S. Pat. No. 6,270,221 to Liang et al. Liang et al had difficulty relying upon the human eye to focus the incoming large light beam into a small spot on the retina, due to the shortcomings of the Hartmann Shack lenslet system. The very thing that made the Hartmann Shack device useful (measuring eyes with problems focusing), gave it trouble with those eyes. Eyes that did not focus well could not be measured because the reflected wavefront did- not originate from a small spot, it originated from a large spot, degrading the performance of the lenslets. Because the Hartmann Shack system uses lenslets, it is very sensitive to this type of error. The Liang et al solution was to add focusing lenses to converge or diverge the illumination beam to compensate for the refractive deficiencies of the eye, as well as the extreme sensitivity of the Hartmann Shack lenslet system to this problem.
Applicant""s invention is an improvement on the Horwitz system.
With respect to Horwitz, the light beam projected into the eye, in Applicant""s system, is of a diameter much less than the diameter of the eye""s pupil. The Horwitz system required that the eye""s cornea and lens focus the light into a small point on the surface of the retina. When a patient""s eye was working well, a small point of light did form on the retina. However, if a patient""s eye was not working well, or was simply accommodating, a small point of light was not formed. Instead, a larger spot of light was formed. The worse the refractive condition of the eye, the larger the spot became. This resulted in light being reflected from the retina from many points, which degraded the image quality of the fringes. The addition of a spatial filter and screen helped filter out many of the unwanted reflections, but not all. More importantly though, the spatial filter placed a limit on the :measurement range of the device, and the screen reduced its sensitivity to some higher order aberrations. Reduced sensitivity to these higher order aberrations may have been acceptable, and even desirable at the time of the Horwitz invention, but now it is desirable to measure and quantify them. Additionally, the measurement range of the previous device was acceptable in its time, but now higher measurement range demands are being placed on these systems, and more range is required.
By using a beam diameter much smaller than the eye""s pupil diameter, such as less than 1 mm, the beam passes through the central axis of the eye, where virtually no refraction takes place. Regardless of the optical performance of the patient""s eye, or its accommodative state, the light still forms into a small spot on the retina, which results in a much better quality return signal for purposes of fringe pattern generation. Small illumination beam diameters may also be projected into the eye from other angles and positions, so long as they impinge upon the retina at the point corresponding to the central optical axis.
No topography or pachymetry is employed. Applicant""s invention only analyzes the light reflected from the retina and refracted through all the optical components of the eye. It does not analyze light reflected from other surfaces such as the cornea and crystalline lens. No eye tracking is performed. Unlike the Horwitz system, no spatial filter or screen is used.
With respect to the Williams system, Applicant""s invention does not use a deformable mirror to modify the light being projected into the eye, nor does it use a deformable mirror to modify the reflected light being collected from the eye. Deformable mirrors are quite complex, adding significant cost to the Williams system. Applicant""s system is simpler, lower in cost, more robust and simpler to service and maintain. Applicant""s system does not use a lenslet array. It utilizes instead a reticle, a completely different optical method than the lenslet array. The lenslet array condenses and converts the wavefront into spots, whereas the reticle preserves the wavefront and images it by introducing contrasting dark and light lines within it. The Williams"" spots indicate the wavefront shape, whereas Applicant""s dark lines indicate the wavefront shape. As a result, Applicant""s system can measure both fine, medium and coarse aberrations, whereas the Williams system can only measure medium aberrations. Two limitations of the Hartmann-Shack (HS) aberrometer required by Williams are: measuring coarse aberrations and measuring fine aberrations. Williams"" underlying assumption is that the wavefront is locally flat, and each lenslet cleanly focuses the collected light into a small spot. The position of this spot is then measured as to how far it deviates from the lenslet""s optical axis, and a determination can then be made as to the error in the wavefront being collected. This assumption breaks down when the lenslet collects light that has coarse aberrations because the light cannot be focused. into a small spot. Significant curvature of the wavefront causes a blurry spot, making it difficult to measure the position of the spot""s central location. This assumption again breaks down when the lenslet collects light that has fine aberrations because again, the light cannot be focused into a small spot.
Also as a result of using a lenslet array, the Williams"" system cannot see the eye being evaluated within the image that the lenslet array produces, it can only see an approximate position. Williams requires a second camera to see the eye being evaluated, not only adding complexity, but introducing potential measurement error. Applicant""s system can see the eye within the image produced by the reticle(s). This not only results in a simpler system because only one camera is required, it also guarantees that the wavefront measurement is accurately matched to the location of the pupil.
Because of the use of a lenslet array, Williams cannot accurately locate the glint (purkinje image) within the image that the lenslet array produces, but Applicant""s system can see the glint within its image. The observation of, and the accurate identification of the location of this point of light is essential in ensuring that the patient""s eye being measured is gazing directly into the wavefront measurement device. Gaze angle misalignment will result in the inadvertent incorrect measurement of the optical center of the eye. The resulting corrective action being taken on the eye, surgery or corrective lenses, will produce the extremely undesired result of the correction being de-centered.
A further advantage of using reticle(s) rather than lenslet arrays is that more robust means (Fourier Analysis), can be used to analyze the signal, making the system not only less susceptible to signal loss, but more reliable in the mathematical calculations. One of the reasons for this advantage is that Applicant""s invention is not required to locate the central spot of each single image formed by each single lenslet. Instead the image is converted into the frequency domain, allowing far more opportunities for the application of mathematical analysis algorithms, described in greater detail later in the specification.
With respect to Liang et al, Applicant""s invention not only differs significantly in that no lenslet array is used, but it also does not use focusing means to condition the illumination beam. In addition to the aforementioned problems of using lenset arrays, using such focusing means adds complexity and cost to the system, as well as adding time to make the measurement because of the added steps required to adjust the incoming beam to match the properties of the patient""s eye.
It is an object of the present invention to provide a system which will measure aberrations in the eye to determine the prescription required to correct for defects.
It is also an object of the invention to use two devices simultaneously to provide a system to allow the investigation of both eyes simultaneously.
It is a further object of the invention to provide a system to measure the eye which can be connected to a device to make Intraocular Lenses (IOLs), contact lenses or custom spectacle lenses.
It is yet a further object of the invention to provide a system to measure the eye which can be connected to a device to guide Laser Surgery.
Yet another object of the invention is to provide a system to measure aberrations with the patient""s glasses or contacts on, to test/screen if the prescription is correct.
Still another object of the invention is provide a system to measure the focus of non-verbal children, with and without glasses; measure for IOL prescriptions once a human lens is out of the bag, but before an artificial IOL is inserted and to measure refraction as the IOL is being tuned in the eye.
These and other objects of the invention will be seen from the detailed description of the invention that follows.